This invention relates to liquid crystal devices, and more particularly to nematic liquid crystal display assemblies.
The use of nematic liquid crystal materials and compositions for electro optical modulation of light, as in display assemblies for generating alpha-numeric characters, is well known. Typical devices of this nature and suitable nematic liquid crystal materials, therefore, can be found described in U.S. Pat. Nos. 3,625,591, 3,655,269 and 3,655,270. Illustrative of such nematic liquid crystals is p-methoxybenzylidene-p-n-butylaniline (also, commonly referred to as MBBA).
Heretofore, it is accepted in the art (as for example see U.S. Pat. No. 3,503,672) that ionic conduction is necessary to bring nematic liquid crystals into the scattering regime, with associated inclusion of appropriate additives to decrease the specific resistance of the liquid crystal cell to about 10.sup.8 -10.sup.9 ohm-cm. However, it has been found that at these levels of conductivity, reactions occur between the electrodes and such nematic liquid crystals which limit the useful life of display cells to a few hours in D.C. operation, or to a few weeks in square wave operation, with the electrodes often showing severe electrolytic corrosion.
Normally, it could be assumed that such corrosion could be constrained by protection of the metal electrodes with an insulating film in view of teachings of the nature disclosed in U.S. Pat. Nos. 3,257,903 and 3,600,060. In the first patent, a device is described utilizing dipole particles in which the side of an electrode disposed adjacent the particles is said to be covered by a transparent protective coating such as silicon monoxide. In the latter patent it is indicated that in a preferred mode, it is advantageous to use transparent insulating layers of suitable materials, such as glass and plastic, as a protective layer on top of transparent electrodes in cholesteric liquid crystal display devices. However, as will be observed, such units employ materials which are merely field sensitive without need of ionic conduction to bring them into a scattering mode.
Such an approach has not been seriously considered in the art for nematic liquid crystal devices since all of the applied field will appear across the thin insulator and thus not bring the nematic liquid crystal in the scattering mode. Also, as is well known, liquid crystal display assemblies generally employ electrodes comprised of an image portion and a lead portion for connection to a suitable signal means for generation of an electric field across the cell, or for an example see the above referenced U.S. Pat. No. 3,655,270. In operation of such a configuration, under an applied field, the image and lead portions or segments of the electrode, both activate the nematic liquid crystal material in the scattering mode with corresponding visual representation of each seen in display. To minimize the effect of the lead line, under an applied signal, its width is normally reduced to minimum dimensions acceptable for conduction of an electrical signal to the image segment of the electrode. Thus, as will be appreciated, there is a limit to which the width of the lead portion of the electrodes can be reduced for proper signal conduction.